A field study of the behavior, ecology, social organization, and vocal communication of the rare lion-tailed monkey (Macaca silenus), the most arboreal macaque, is underway in the mountainous rain forests of South India. Troops ranging in size from four to over thirty members are under observation in dense primary forest interspersed with relatively open sectors produced by removing the understories for cardamom plantation. Emphasized is relating a) intertroop variation in behaviors and ecology to group size and b) transient intratroop differences in socioecology to habitat structure including seasonal variation in abundance and distribution of foods. Data are recorded systematically on activities, spacing, movements, heights, and habitat utilization, yielding results expressed as activity profiles, diurnal cycles, social interaction networks, interindividual associations, range utilization, feeding ecology, and the differentiation of these by the age and sex class of the animals. The effect of forest structure and visibility-limiting fog on vocal behavior are being examined. A status survey and census throughout their range will be performed as an aid to recommending conservation measures. A comparative study with the sympatric Macaca radiata and Presbytis johni is planned. The project is in collaboration with the Bombay Natural History Society and carries Government of India approval.